1. Field of the Invention
The present invention relates to an apparatus for enhancing a picture quality for a television receiver, and more particularly to an improved gamma correction circuit for a television receiver which makes it possible to compensate for a picture quality degradation and a detail aggravation of a reproduced on-screen image that occur in accordance with a transmitter's gamma correction.
2. Description of the Prior Art
Recently, as a television receiver tends to be bigger, more deluxe and multi-functional, diversified studies are being made to accomplish objectives concerned, such as high luminance, high definition, noise elimination, and high clearness.
However, despite such efforts, there still remains much room for more improvement with regard to fundamental factors that may cause picture quality degradation. Further, there is required a study as to a method of color reproduction using a quantitative analysis.
In a presently adopted NTSC (national television system committee) method, a picture obtained from an RGB (red-green-blue) coordinate system is converted to a YIQ coordinate system, for thereby being transmitted as a color difference signal, and a television receiver converts the transmitted color difference signal to an RGB signal so as to reproduce a color picture.
When a color reproduction is achieved in an NTSC color television system, there occur a variety of color reproduction errors depending upon characteristics of a receive/transmit circuit for sending an original picture, a modulation/demodulation circuit, and a television receiver.
As shown in FIG. 1, a CPT (color picture tube) adopted in the NTSC color television system is provided with a gamma characteristic in which a luminance signal y serving as a light-emission output value of the television receiver is non-linearly proportional to a y square of a video signal x, for thereby obtaining an equation y=kx.sup..gamma. in a light-emitting zone. The CPT is also provided with a non-linear characteristic in which a light-emission does not occur with regard to a negative input signal.
So, in order to compensate for such a non-linear characteristic of a television receiver under a current NTSC television system, there is carried out a .gamma. correction in which the non-linear characteristic of the RGB signal is compensated for in the television receiver before the RGB signal is converted to the YIQ signal.
With reference to FIG. 1, a conventional NTSC TV system is provided with: a camera 10 for linearly obtaining a brightness data of an object material and outputting an original color RGB signal; a gamma correction unit 11 for applying a gamma correction to the original RGB signal outputted from the camera 10; a matrix 12 for converting the original RGB signal outputted from the gamma correction unit 11 to a color difference YIQ signal; a zone confinement unit 13 for confining a frequency zone of the color difference YIQ signal outputted from the matrix 12; an adder 14 for adding the output value of the zone confinement unit 13 to an distortion value d of a transmission channel; an inverse matrix 15 for converting the output of the adder 14 to original color RGB signal; and a CPT 16 for displaying the original color RGB signal outputted from the inverse matrix 15.
The operation of the thusly constituted conventional NTSC TV system will now be described with reference to the accompanying drawings.
The camera 10 linearly obtains a brightness data of an object material and outputs a picture data X serving as the output value E.sub.R,E.sub.G,E.sub.B of the camera 10, or a transmission channel signal. The picture data X may be expressed as a vector with regard to a luminance signal and a pair of color signals as shown below: ##EQU1##
The gamma compensation unit 11 receives the picture data X and applies a gamma correction X.sup..GAMMA. to the original color RGB signal so as to compensate for a non-linear characteristic of a television receiver, and the matrix related thereto is as the following: ##EQU2##
The matrix 12 receives the original RGB signal compensated in the gamma correction unit 11, and it is multiplied by a conversion coefficient M so as to obtain [M*(X).sup..GAMMA. ], converted into a luminance signal Y and a color difference signal IQ, and transmitted through a transmission channel. EQU .PSI.(X)=0.3X.sub.1 +0.59X.sub.2 +0.11X.sub.3 EQU I(X)=0.7X.sub.1 -0.59X.sub.2 +0.11X.sub.3 EQU Q(X)=-0.3X.sub.1 +0.59X.sub.2 +0.89X.sub.3
At this time, the conversion coefficient M may be expressed as the following matrix: ##EQU3##
The zone confinement unit 13 serving as a transmission channel confines the respective zones of the luminance signal .PSI. and color difference signals I, Q to 0.5 Mhz or 1.5 Mhz, and the channel frequency characteristic F in the zone confinement unit 13 is expressed as below: ##EQU4##
Here, E denotes a unit matrix, and f.sub.c denotes a color signal breaking frequency.
Also, the adder 14 adds a distortion value d of the transmission channel to the output of the zone confinement unit 13, and the resultant value is outputted through the channel.
The receiver side matrix 15 converts to the original color RGB signal the luminance signal Y and the color difference signals I, Q which are transmitted through the transmission channel so as to be outputted to the CPT, wherein the process is carried out by reverse-converting the conversion coefficient M of the matrix 11.
At this time, the reverse-conversion in the receiver side may be expressed as M.sup.-1 =(I.sub.ij), which denotes an inverse matrix of a matrix M=(m.sub.ij) in the transmitter side.
The CPT 16 serves to output a color signal X.sub.D and a luminance signal Y.sub.D as shown below: EQU X.sub.D =[M.sub.1 (F*M*(X).sup..GAMMA. +d)].sup..gamma. EQU Y.sub.D =.PSI.(X.sub.D)
Here, d denotes a distortion value of the transmission channel, and .gamma. is equal to 1/.GAMMA..
The picture data X outputted from the camera 10 may be expressed as a sum obtained by adding a low frequency component X.sub.L and a high frequency component X.sub.H under a criterion of a color signal breaking frequency f.sub.c for thereby turning out an expression X.sub.i =X.sub.Li +X.sub.Hi.
At this time, assuming that the high frequency component is less than the low frequency component, the output Xr is as shown below: EQU X.sub.i.sup..GAMMA. =X.sub.Li.sup..GAMMA. +.GAMMA.X.sub.Hi .multidot.X.sub.Li.sup..GAMMA.-1
The output X.sub.D of the CPT 16 realized in the currently adopted NTSC system is as shown in an expression (1), in which the high frequency component X.sub.H which is larger than the color signal breaking frequency f.sub.c is distorted by a gamma correction in the transmitter side: EQU X.sub.D =[X.sub.L.sup..GAMMA. +.GAMMA..PSI.(X.sub.Hi X.sub.L.sup..GAMMA.-1)].sup..gamma. =X.sub.L +X.sub.L.sup.1-.GAMMA. .PSI.(X.sub.Hi X.sub.L.sup..GAMMA.-1) (1)
The luminance signal Y.sub.D outputted from the CPT 16 may be incorporated into a low frequency component Y.sub.L and a high frequency component Y.sub.DH, wherein the luminance signal Y.sub.D may be expressed as an expression (2) and the high frequency component Y.sub.DH may be expressed as an expression (3): EQU Y.sub.D =.PSI.(X.sub.D)=Y.sub.L +Y.sub.DH (2) EQU Y.sub.DH =.SIGMA.m.sub.1k X.sub.Lk.sup.1-.GAMMA. .SIGMA.m.sub.1i X.sub.Hi X.sub.Li.sup..GAMMA.-1 (3)
Due to the gamma correction in the transmitter side with reference to the expression (3), the low frequency component Y.sub.L of the luminance signal Y.sub.D which exists below the color signal zone becomes correctly reproduced, however, the high frequency component Y.sub.DH becomes distorted in accordance with a gamma correction in the transmitter side.
At this time, the proportion P in which a luminance signal is reproduced may be expressed as an expression (4): ##EQU5##
That is, when a picture is colorless, P=1 becomes satisfied so as to represent that the luminance signal is correctly reproduced. For instance, when only a high chromatic green picture is transmitted, the proportion P of the luminance signal being reproduced becomes 20 dB as shown in an expression (5): ##EQU6##
FIG. 2 exhibits brightness of the high frequency component Y.sub.DH with regard to the luminance signal Y.sub.D reproduced in the CPT 16 when a color becomes converted to a more chromatic color picture such as red, green and blue, as shown therein, a detail degradation becomes significantly larger in a high chromatic color picture. That is, the graph in FIG. 2 denotes the high chromatic detail degradation according to the gamma correction in the transmitter side.
Meanwhile, the proportion P in which a luminance signal is reproduced is a six-variable function between X.sub.L and X.sub.H, so that the proportion P does not become a value 1. When there is provided a high frequency component in the expression (5), the high frequency component of a reproduced luminance with regard to an object color may be expressed as an expression (6): ##EQU7##
Also, when there exist high frequency components for red, green and blue (X.sub.H1 .apprxeq.X.sub.H2 .apprxeq.X.sub.H3), the high frequency components of the reproduced luminance with regard to the luminance component of the object color may be expressed as an expression (7), wherein the reproduced brightness with regard to the high frequency components is larger that of the camera 10 output. ##EQU8##
As described above, in the case in which a picture that is to be reproduced is of a high luminance picture, the brightness data being transmitted in accordance with the color difference signals I, Q is larger than the brightness of the luminance signal, and the zone for color signals confined to 0.5 Mhz or 1.5 Mhz, so that a brightness data higher than that of the breaking frequency becomes lost, whereby there occurs a significant picture quality degradation in the picture reproduction.
In order to improve such picture degradation, an EDTV in Japan employs a gamma correction circuit 20 in a transmitter side as shown in FIG. 3.
At this time, the gamma correction circuit 20 is focused on what extent of a high frequency luminance signal should be sent to the transmitter side so as to correctly reproduce a high frequency signal in the receiver side.
That is, the matrix 20-1 of the gamma correction circuit 20 receives color signals R, G, B from the camera 10, and it is multiplied by a conversion coefficient M of the transmitter side and converted into a luminance component Y. The converted luminance component Y serving as a low zone component is eliminated from a high pass filter 20-2, whereby only a luminance signal Y.sub.B serving as a high zone component is provided to a divider 20-4.
The divider 20-4 receives a color signal X.sub.r through a zone confinement unit 13 and an amplifier 20-3, and the high zone component luminance signal Y.sub.H is divided by the received color signal X.sub.r, for thereby obtaining a compensated luminance signal h.
That is, the signal X.sub.R modulated in the transmitter side may be expressed as an expression (8) in the receiver side: ##EQU9##
Here, if h &lt;&lt;X.sub.L is satisfied, the luminance signal Y.sub.D outputted from the CPT 16 may be expressed as an expression (9): EQU Y.sub.D =.PSI.(X.sub.L)+.gamma.h.PSI.(X.sub.L.sup.1-.GAMMA.)(9)
In order for the luminance signal Y.sub.D in the expression (9) to be proportional to a brightness signal of an object, the value h should satisfy an expression (10): EQU h=Y.sub.H .vertline..gamma..PSI.(X.sub.L.sup.1-.GAMMA.) (10)
Here, assuming that the gamma value in the CPT 16 is "2", h serving as an output value of the gamma correction circuit 20 may be expressed as an expression (11): EQU h=Y.sub.H .vertline.2Y.sub.L (11)
As a result, the gamma correction circuit 20 is focused on what extent of the luminance signal of the high frequency zone in the transmitter side should be transmitted in order to correctly reproduce the high frequency signal, for thereby correcting the high frequency zone of the luminance signal.
However, the conventional gamma correction has a disadvantage in that, an additional correction circuit should be realized in the transmission system side which serves to transmit a TV signal, and further a standard for the currently employed NTSC system should be transformed.